Drilling apparatus with percussive action cutter

ABSTRACT

A drilling apparatus includes a cutting structure defined by a percussive action cutter and at least one other cutter. The percussive action cutter is adapted to be urged in a drilling direction by hydraulic pressure force, while the other cutter is urged in a drilling direction by mechanical force.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/488,216, which was filed Aug. 31, 2004, and which is incorporatedherein by reference. This application and the Ser. No. 10/488,216application claim the benefit of priority of each of InternationalApplication No. PCT/GB02/02575, filed Jun. 5, 2002 and published as WO02/099242 A1 on Dec. 12, 2002, United Kingdom (GB) Application No.0113585.4, filed Jun. 5, 2001, and United Kingdom (GB) Application No.0114874.1, filed Jun. 19, 2001.

FIELD OF THE INVENTION

Embodiments of the invention relate to drilling apparatuses, and inparticular to drilling apparatuses for use in drilling bores in earthformations.

BACKGROUND OF THE INVENTION

Conventional drill bits, as used for example in the drilling of bores inthe oil and gas exploration and production industry, feature a number oftoothed roller cones. In use, weight is applied to the rotating bit, andthe cones roll over the circular face forming the end of the bore. Thecutting of the rock is achieved by a number of mechanisms, including acrushing action, the crushed rock then being removed from the cuttingface by the action of drilling fluid exiting the drill bit viaappropriately directed jetting nozzles. In addition, by varying theorientation of the rotational axes of the cones, it is possible toprovide a scraping or scouring action as the cones rotate.

A more recent development, allied to advancements in drilling andmaterials technology, has lead to the development of drill bitsfeaturing fixed cutting faces provided with relatively hard materials,typically polycrystalline diamond compact (PDC).

Traditionally, drilling of the deep bores required in the oil and gasindustry relied solely upon applied weight, which weight may be appliedfrom surface, or achieved simply by the mass of the drill string, drillcollars and other tools and devices above the bit. However, in recentyears drilling has commenced in areas featuring particularly hard rock,in which drilling using conventional methods is still possible, butrelatively slow. In attempts to overcome this problem, there have beenvarious proposals for percussive or hammer drill bits; while air-poweredhammer drills are in common use in, for example, the construction tradesand in mining, drilling of deep bores with non-compressible drillingfluid poses many different and varied problems. One difficulty has beenin reliably achieving the necessary impact forces at the bit. Thelimited diameter available downhole limits the size of the hammer tool,resulting in excessive loading and wear on the tool in order to achievethe required impact force. Another difficulty that has been experiencedin percussive bits is the tendency for the bits to “lose gauge”, that isthe outer edges of the bits wear prematurely, with the result that thebit drills a smaller diameter bore than intended, or must be replaced atfrequent intervals.

SU 1730420 A1 (Leningrad Scientific-Research & Design Institute“Gipronikel”) discloses a combined percussive rotary drilling tool. Inaddition to a conventional roller-cutter, the tool features a centralpercussive bit which cooperates with a piston actuated by a supply ofcompressed air. Clearly, such an arrangement would not be suitable indownhole operations where there is no supply of compressed airavailable; conventionally, drilling “mud” is supplied to the drill bit,which liquid could not be utilised to operate a piston as disclosed inthis document.

U.S. Pat. No. 3,807,512 discloses a percussion-rotary drilling mechanismwith a rotary drill bit and a percussion drill bit. A mud drive turbineis utilised to generate reciprocal motion of the percussion drill bitvia an arcuate cam and cam follower drive mechanism or a rotatableeccentrically weighted wheel drive mechanism.

BRIEF SUMMARY OF THE INVENTION

It is among the objectives of embodiments of the present invention toprovide an improved drilling apparatus which provides or at leastfacilitates a greater degree of control in a drilling apparatus having apercussive action cutter provided in combination with another form ofcutter.

According to a first aspect of the present invention there is provideddrilling apparatus comprising a cutting structure defined by at leastone percussive action cutter and at least one other cutter, the at leastone percussive action cutter being adapted to be urged in a drillingdirection by hydraulic pressure force.

The combination of a percussive action cutter with a cutter of anotherform, such as a roller cone or a fixed cutter, such as a PDC cutter,offers many advantages over conventional drilling apparatus and drillbits. For example, the hammer action of percussive bits is mosteffective working with relatively light weight applied to the bit,however this may limit the cutting effectiveness of the bit, such thatin conventional hammer bits a balance must be struck between these tworequirements. However, in the present invention the other cutter maybear a significant proportion of the weight applied to the bit, allowingthe percussive action cutter to operate more effectively. Indeed, inpreferred embodiments of the present invention, substantially all of themechanical force normally applied to the apparatus, whether by forceapplied to a drill string from surface or due to the mass of drillcollars, the drill string and the like, is applied to or borne by theother cutter. The “weight” applied to the percussive action cutter is afunction of applied hydraulic pressure, equivalent to the “pump-openforce”, and thus may be controlled independently of the weight appliedto the other cutter, and solely with a view to maximising theeffectiveness of the percussive action cutter. This division of forcebetween the cutters also serves to utilise the different forcesavailable, that is mechanical force and hydraulic force, in an efficientand effective manner.

Furthermore, as the percussive action cutter only provides a proportionof the area of the cutting face, for a given available hammer orpercussive force the impact pressure force applied by the relativelysmall area percussive cutter to the rock may be relatively high. Thiseffect may be further accentuated by the ability to create a hammer orpercussive action within the body of the apparatus over an area which isrelatively large when compared to the cutting area of the percussiveaction cutter. In conventional percussive drill bits, the hammer tooldiameter is always substantially smaller than the cutting area of thebit, as the bit has to cut a bore of a gauge large enough to accommodatea drilling fluid return annulus and the body of the tool. In embodimentsof the present invention the hammer tool may be at least as large as thecutting area. Of course other embodiments of the invention may achieve ahammer or percussive effect in other ways, as will be apparent to thoseof skill in the art.

Preferably, the apparatus includes means for creating a percussive forceand means for transferring the resulting percussive force to the atleast one percussive action cutter. Typically, the means for creating apercussive force will comprise a hammer tool, and the percussive actioncutter will define an anvil. Conveniently, the hammer tool will behydraulically actuated, although other forms of actuation may beemployed if desired.

Preferably, the percussive action cutter defines a flow restriction,such that hydraulic fluid flowing therethrough experiences a pressuredrop, and thus creates a pressure force on the cutter. The restrictionmay take any appropriate form, and will typically be provided by one ormore jetting nozzles. The flow restriction may be provided incombination with a piston area, which piston area may also serve as ananvil.

Preferably, there is a degree of overlap in the area swept by thecutting surface of the percussive action cutter and the other cutter.Thus, if there is a loss of gauge through wear to the percussive actioncutter, this will be accommodated by the other cutter.

The percussive action cutter may be located to cut a central portion ofthe bore. Due to the relatively low speed of a rotating drill bit at thebit centre, there are often difficulties experienced in cutting thecentre of the bore. Thus, by locating the percussive action cuttercentrally, the enhanced cutting action provided by the hammer drilleffect will avoid this difficulty. Furthermore, it is generallydesirable to rotate percussive action cutters at relatively low speed(10 to 40 rpm), the primary reason for rotation being to expose freshformation to the individual cutter elements, with higher speeds leadingto excessive or accelerated wear of the cutting elements. Merely bylocating the percussive action centrally, the linear speed experiencedby the cutter elements is of course relatively low in comparison to theother cutters located radially outwardly of the percussive actioncutters, which other cutters typically benefit from higher cuttingspeeds (150 to 200 rpm). Alternatively, or in addition, the percussiveaction cutter may be biassed rearwardly, conveniently by means of aspring, or otherwise configured, such that the percussive action cutteris normally held slightly off bottom and thus remains in contact withthe formation only for the duration of the hammer impact or impulse. Thepercussive action cutter will thus only make contact with the formationperiodically, and for only a fraction of the time the other cutterremains in contact with the formation. This will reduce the rubbingaction and wear experienced by the percussive action cutter, even athigher rotary speeds, allowing the apparatus to be rotated at speedssuited to the other cutter without damaging the percussive actioncutter.

A centrally located percussive action cutter may also have a cuttingface located forwards of the cutting face of the other cutter, such thatthe percussive action cutter effectively cuts a pilot hole in the centreof the bore. Of course this facilitates the dislodgement of rock by thefollowing cutter. Furthermore, by providing lateral fluid outlets on thepercussive action pilot cutter, ahead of the other cutter, drillingfluid may be injected into the rock formation ahead of the other cutter,facilitating the release of cuttings by the following cutter.

In light of the possibility of the different forms of cutter achievingdifferent cutting speeds, the apparatus may include one or more of thefollowing features.

Means may be provided for indicating that the other cutter is cutting ata faster rate than the percussive action cutter, allowing, for example,the weight applied to the other cutter to be reduced, thus reducing thecutting speed of the other cutter, improving the cutting efficiency ofthe percussive action cutter, and preventing premature damage and wearto the percussive action cutter, which may have been experiencingexcessive applied weight. Such means may take the form of fluid outletswhich are closed if the percussive action cutter experiences elevatedweight and is forced rearwardly into the body of the apparatus. Theresulting change in back pressure will be detectable at surface,allowing remedial action to be taken.

In circumstances where the percussive action cutter is cutting fasterthat the other cutter, extension of the cutter beyond a predeterminedrelative position may result in the associated percussive tool ceasinghammering, allowing the other cutter to catch up; it is a standardfeature in many percussion tools that if the tool is picked up offbottom the tool ceases hammering. Alternatively, in a hammer and anvilarrangement the anvil on the cutter may simply move beyond the end ofthe hammer stroke.

A centrally located percussive action cutter may be retractable orremovable, to allow the cutting of cores by the remaining cutter, toallow passage of other tools or devices through the apparatus, or tofacilitate flow of, for example, cement slurry, through the apparatus.Thus, the drilling apparatus may be utilised as a casing shoe. In suchapplications it is likely that the shoe will be provided with fixedreaming cutters, typically PDC cutters, which tend to require a highapplied torque to rotate the cutters to ream out obstructions to thepassage of the shoe and following casing; however, casing, and casingthreads, tend not to be capable of accommodating elevated torques. Byproviding a percussive action cutter in the shoe, centrally or otherwiselocated, the torque required to rotate the shoe may be reduced.

According to another aspect of the present invention there is provided adrilling method comprising the steps:

providing drilling apparatus comprising a cutting structure defined byat least one percussive action cutter and at least one other cutter; and

urging the least one percussive action cutter in a drilling direction byhydraulic pressure force.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

These and other aspects of the present invention will now be described,by way of example, with reference to the accompanying drawings, inwhich:

FIG. 1 is a sectional view of a part of a drilling apparatus inaccordance with a first embodiment of the present invention;

FIG. 2 is a view from below of the apparatus of FIG. 1;

FIG. 3 is an alternative sectional view of the drilling apparatus ofFIG. 1;

FIGS. 4 and 5 are sectional views of a part of a drilling apparatus inaccordance with a second embodiment of the present invention;

FIG. 6 is a sectional view of a part of a drilling apparatus inaccordance with a third embodiment of the present invention; and

FIG. 7 is a sectional view of a part of a drilling apparatus inaccordance with a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference is first made to FIGS. 1, 2 and 3 of the drawings, which showa drilling apparatus 10 in accordance with a first embodiment of thepresent invention. The apparatus 10 comprises a bit 12 mounted via a pinand box connection 14 to the lower end of a hammer tool 16.

Mounted on the lower end of the drill bit are two roller cones 18, 19;when the bit 12 is rotated, the cones 18, 19 will roll over the rockformation below the bit 12, crushing or otherwise dislodging cuttingsfrom the rock. The cutting structure of the bit 12 is further defined bytwo circular cutting faces defined by the ends of a pair of percussiveaction cutters 20, 21. As may be seen from FIG. 2, the roller cones 18,19 are positioned on opposite sides of the bit 12, and the percussiveaction cutters 20, 21 are located in the quadrants between the rollercones 18, 19.

Both percussive action cutters 20, 21 are mounted on the end of amandrel 22 which extends through the bit body and into the hammer tool16. The mandrel 22 is splined, at 24, and co-operates with the bit body13 such that the mandrel 22 may move axially relative to the body 13,but cannot rotate relative to the bit 12. An upper portion of themandrel 22 defines an annular recess 26 which co-operates with a ring 28trapped between the hammer tool body 29 and the bit body 13, to limitthe axial movement of the mandrel 22.

The upper end face of the mandrel 22 defines an anvil 30 against whichimpacts a hammer 32. Reciprocal movement of the hammer 32 may beachieved by any appropriate means, and may utilise pressure forcescreated by the pressure of drilling fluid within the drill string. Thedrilling fluid passes through the apparatus 10 and exits the apparatusthrough various appropriate jetting nozzles adjacent the cutters 21, 22and the roller cones 18, 19. The drilling fluid passes through themandrel 22 and, with the pressure drop resulting from the fluid passingthrough the nozzles, creates a pressure force which acts on the upperarea of the anvil 30 and thus urges the cutters 21, 22 in a drillingdirection with a force proportional to the drilling fluid pressure.

In use, the apparatus 10 will be run downhole on the end of an otherwiseconventional drillstring. Drilling fluid will be pumped from surface andwill travel through the string, the hammer tool 16 and the bit 12, toexit the bit through appropriate jetting nozzles (not shown in FIG. 1 to3), as noted above. The bit 12 is rotated such that the roller cones 18,19 are rolled over the end face of the bore, cutting the rock formationtherebelow in a conventional manner. However, the impact of the hammer32 on the anvil 30 will produce a percussive or hammer action which istransferred via the mandrel 22 to the percussive action cutters 20, 21.Thus, as the drill bit 12 rotates, the cutters 20, 21 are urged into therock formation with an impact force.

It is believed that the combination of the cutting action provided bythe roller cones 18, 19 and the percussive hammer action of the cutters20, 21 will be more effective than the action of, for example, a drillbit featuring only roller cones, particularly when drilling throughrelatively hard formations. Furthermore, the mechanical weight appliedto the bit 12 will be supported by the roller cones 18, 19, such thatthe applied weight may be relatively high without detracting from thehammer action of the cutters 20, 21: as noted above, the “weight”applied to the cutters 20, 21, and on top of which the impact or impulseis applied, is a function of the hydraulic pressure of the drillingfluid, and may be controlled independently of the applied mechanicalweight to provide efficient percussive drilling. Furthermore, thecutting area of the percussive action cutters 20, 21 is relatively smallwhen compared to the size of the hammer tool 16, such that thepercussive or impact pressure force applied to the rock formation willbe relatively high.

Reference is now made to FIGS. 4 and 5 of the drawings, whichillustrates drilling apparatus 40 in accordance with a second embodimentof the invention. The apparatus 40 shares a number of features with theapparatus 10 described above, however in this apparatus 40 only a singlepercussive action cutter 42 is provided, and the cutter 42 is locatedcentrally of the drill bit 43. Thus, in the course of a drillingoperation, the percussive action cutter 42 will cut a pilot bore, andthe following roller cone cutters 44, 45 will effectively provide areaming operation to bring the bore out to gauge.

The cutter 42 is mounted on the lower end of a mandrel 46, the upper endof which defines an anvil 48 which is struck by the hammer 50 of thepercussive tool 52 provided above the apparatus 40. The tool body 54defines a bore 56 and the anvil 48 features a seal 58 which provides asliding seal between the bore 56 and the anvil 48; as with the previousembodiment, the “weight” or force normally applied to the cutter 42 istherefor dependant on the internal fluid pressure acting on the sealarea of the anvil 48.

The figures also illustrate a central bore 60 passing through themandrel 46, the bore 60 leading to appropriately located jetting nozzles62, 64. In use, the jetting nozzles 62 located in the face of thepercussive action cutter 42 will inject pressurised drilling fluid intothe walls of the pilot bore created by the cutter 42, thus facilitatingcuttings removal by the cones 44, 45. If desired, jetting nozzles may beprovided on the sides of the cutter 42, such that drilling fluid isdirected laterally of the cutter 42, directly into the surroundingformation.

The other jetting nozzles 64 are directed towards the cones 44, 45, andare normally located below the end of the bit body bore 66, which formsa continuation of the hammer tool body bore 56. However, if the cuttermandrel 46 moves rearwardly into the bit body 68, the nozzles 64 areclosed, and which may be identified at surface by an increase in theback pressure of the drilling fluid.

The presence of the percussive action cutters 42 in the centre of thebit avoids the difficulties normally associated with drilling the centreof the bore using a conventional bit. Furthermore, in the event of lossof gauge of the cutter 42, it is apparent from FIGS. 4 and 5 that such areduction in diameter of the cutter 42 would have no significant effect,as the swept area of the roller cones 44, 45 overlaps the outer edge ofthe cutter 42.

Furthermore, if the cutting rate of the cones 44, 45 should exceed thecutting rate of the cutter 42, and the cones 44, 45 “catch up” with thecutter 42, as illustrated in FIG. 5, the action of the hammer tool maybe affected, and the cutter 42 may experience a potentially damagingincrease in applied weight. In this event, as noted above, the nozzles64 are closed and an increase in back pressure will be noted at surface.The operator is therefore alerted to reduce the applied weight, slowingthe cutting speed of the cones 44, 45, for example by reducing themechanical weight applied to the cones 44, 45, and allowing the cutter42 to move ahead of the cones 44, 45, to the normal, optimum drillingposition.

On the other hand, if the cutting rate of the cutter 42 should exceedthe cutting rate of the cones 44, 45, the anvil 48 will move towards oreven beyond the end of the stroke of the hammer 50, such that the energytransferred to the cutter 42 will be reduced, and the cutting rate ofthe cutter 42 will decrease. In other embodiments, the percussion toolmay include a control which stops the hammering action when the reactionweight or force experienced by the tool or tool bit, in the form ofcutter 42, falls below a predetermined level.

Reference is now made to FIG. 6 of the drawings, which illustrates, insection, a view of a part of a drilling apparatus 70 in accordance witha third embodiment of the present invention. The apparatus 70 sharesmany features with the apparatus 40 described above, and additionallyincludes a spring 72 provided between a shoulder on the anvil 74 and thering 76 trapped between the lower end of the hammer tool body 78 and theupper end of the bit body 80. The spring 72 is selected such that thepercussive action cutter 82 is normally held slightly off bottom 84, asillustrated. The cutter 82 thus only contacts the bore bottom 84 whenthe hammer 86 strikes the anvil 74 and drives the bit ahead such thatthe cutter 82 impacts the formation. The cutter 82 is thus touching thebottom of the hole only for the duration of the hammer blow, and whenthe hammer 86 moves away from the anvil 74 the cutter 82 springs backoff the bottom of the hole.

This feature of the apparatus 70 reduces the rubbing action and wearexperienced by the cutter 82, such that the apparatus 70 may safely berotated at a relatively high speed better suited to the cutting actionof the cone cutters 90, 92; typically 150 to 200 rpm, rather than therelatively slow speeds (10 to 40 rpm) normally utilised in percussivedrilling.

Reference is now made to FIG. 7 of the drawings, which illustratesdrilling apparatus 100 in accordance with a third embodiment of thepresent invention. The apparatus 100 shares a number of similaritieswith the embodiments described above, however rather than featuringroller cones, the apparatus 100 is provided with a fixed cuttingstructure provided with polycrystalline diamond compacts (PDCS) 102.

As with the apparatus 40 described above, the apparatus 100 features acentrally located percussive action cutter 104 which will, in use, cut apilot bore ahead of the PDC cutters 102.

Conventional drill bits provided with aggressive PDC cutters normallyrequire application of relatively high torques, which may createdifficulties in certain drilling situations. However, in this embodimentof the present invention, the provision of the percussive action cutter104 will tend to reduce the torque necessary to rotate the apparatus100, as a proportion of the cutting is being performed by the percussiveaction cutter 104, which requires a relatively low input torque.

It will be clear to those of skill in the art that the above embodimentsof the present invention, in which one or more percussive action cuttersare combined with other cutters in a single drilling apparatus, offersnumerous advantages over the prior art.

It will also be apparent to those of skill in the art that the aboveembodiments are merely exemplary of the present invention, and thatvarious modification and improvements may be made thereto withoutdeparting from the scope of the invention. For example, the particularconfiguration of the apparatus may vary from the configurationsdescribed above, and embodiments of the invention may be provided informs other than drill bits, for example as reamers or shoes.

1. A drilling apparatus comprising: a cutting structure defined by atleast one percussive action cutter and at least one other cutter, the atleast one percussive action cutter being adapted to be urged in adrilling direction by hydraulic pressure force; and a surface indicatorto alert that the other cutter is cutting at a faster rate than thepercussive action cutter.
 2. The apparatus of claim 1, wherein aplurality of percussive action cutters are provided.
 3. The apparatus ofclaim 1, wherein the at least one other cutter comprises a roller cone.4. The apparatus of claim 1, wherein the at least one other cuttercomprises a plurality of roller cones.
 5. The apparatus of claim 1,wherein the at least one other cutter comprises a fixed cutter.
 6. Theapparatus of claim 1, wherein the at least one other cutter comprises aplurality of fixed cutters.
 7. The apparatus of claim 5, wherein thefixed cutter is a PDC cutter.
 8. The apparatus of claim 1, wherein, inuse, the at least one other cutter is adapted to support at least themajority of mechanical weight applied to the apparatus.
 9. The apparatusof claim 1, further including means for creating a percussive force andmeans for transferring the resulting percussive force to the at leastone percussive action cutter.
 10. The apparatus of claim 9, wherein saidmeans for creating a percussive force comprises a hammer tool.
 11. Theapparatus of claim 10, wherein the means for transferring the resultingpercussive force to the at least one percussive action cutter comprisesan anvil.
 12. The apparatus of claim 10, wherein the hammer tool ishydraulically actuated.
 13. The apparatus of claim 1, wherein thepercussive action cutter defines a flow restriction, such that hydraulicfluid flowing therethrough experiences a pressure drop, and thus createsa pressure force on the cutter.
 14. The apparatus of claim 13, whereinthe restriction comprises at least one jetting nozzle.
 15. The apparatusof claim 14, wherein the flow restriction is provided in combinationwith a piston area defined by the percussive action cutter.
 16. Theapparatus of claim 15, wherein the piston area serves as an anvil. 17.The apparatus of claim 1, further comprising a hydraulically actuatedhammer tool defining a fluid pressure responsive piston area ofdimensions comparable to the cutting area of the at least one percussiveaction cutter.
 18. The apparatus of claim 17, wherein the hammer tool isat least as large as the cutting area of the at least one percussiveaction cutter.
 19. The apparatus of claim 1, wherein there is a degreeof overlap in the area swept by the cutting surface of the at least onepercussive action cutter and the at least one other cutter.
 20. Theapparatus of claim 1, wherein a percussive action cutter is located tocut a central portion of a bore.
 21. The apparatus of claim 1, whereinat least one percussive action cutter has a cutting face locatedforwards of the cutting face of the at least one other cutter.
 22. Theapparatus of claim 21, wherein said at least one percussive actioncutter includes laterally directed drilling fluid outlets ahead of theat least one other cutter.
 23. The apparatus of claim 1, wherein the atleast one percussive action cutter is retractable or removable.
 24. Theapparatus of claim 1, wherein the at least one percussive action cutteris axially moveable relative to the at least one other cutter.
 25. Theapparatus of claim 24, wherein movement of the at least one percussiveaction cutter beyond a predetermined relative axial position is operatordetectable.
 26. The apparatus of claim 25, wherein movement of the atleast one percussive action cutter beyond a predetermined relative axialposition at least restricts fluid flow through a fluid port.
 27. Theapparatus of claim 25, wherein said movement corresponds to said atleast one other cutter cutting ahead of said at least one percussiveaction cutter.
 28. The apparatus of claim 25, wherein forward movementof the at least one percussive action cutter beyond a predeterminedaxial position relative to the at least one other cutter corresponds tosaid at least one percussive action cutter having cut ahead of said atleast one other cutter.
 29. The apparatus of claim 28, furthercomprising means responsive to said forward movement of the at least onepercussive action cutter, said means being effective to at least reducethe percussive action of said at least one cutters in response to saidforward movement.
 30. The apparatus of any of claim 24, wherein the atleast one percussive action cutter is normally retracted from a fullyextended configuration.
 31. The apparatus of claim 30, wherein the atleast one percussive action cutter is spring biased towards the normallyretracted position.
 32. The apparatus of claim 1, wherein the at leastone percussive action cutter comprises a mounting member in slidingsealing contact with a bore defined by a supporting body, whereby fluidpressure in the bore creates a pressure force acting on the cutter. 33.The apparatus of claim 1, wherein the apparatus is in the form of ashoe.
 34. A drilling method comprising the steps: providing drillingapparatus comprising a cutting structure defined by at least onepercussive action cutter and at least one other cutter; urging the atleast one percussive action cutter in a drilling direction by hydraulicpressure force while urging the at least one other cutter in a drillingdirection by mechanical force; and identifying that the percussiveaction cutter is cutting at a faster rate than the at least one othercutter.
 35. The drilling apparatus of claim 1, wherein said surfaceindicator comprises at least one fluid outlet which is closed if thepercussive action cutter experiences elevated weight.
 36. The drillingmethod of claim 34, wherein identifying that the percussive actioncutter is cutting at a faster rate than the at least one other cuttercomprises identifying a change in back pressure in drilling fluid atsurface.
 37. The drilling method of claim 34, further comprising,following identifying that the percussive action cutter is cutting at afaster rate than the at least one other cutter, reducing the weightapplied to the at least one other cutter.
 38. A drilling apparatuscomprising: a cutting structure defined by at least one percussiveaction cutter and at least one other cutter, the at least one percussiveaction cutter being adapted to be urged in a drilling direction byhydraulic pressure force and the at least one other cutter being adaptedto be urged in said drilling direction by mechanical weight, the cuttershaving a preferred relative position range; and a cutter positionindicator providing an output signal if the relative position of thecutters moves outside said preferred range.
 39. A drilling apparatuscomprising: a cutting structure defined by at least one percussiveaction cutter and at least one other cutter; and a percussive actioncutter excess weight warning indicator.